Step motor driving circuit

ABSTRACT

A step motor driving circuit having A-phase and B-phase exciting windings which are independently supplied with exciting current in a sequence of four states per step of the step motor. The driving circuit is capable of applying a large or a small current to each phase winding and the currents are applied in a sequence to result in a trapezoidal shaped exciting current. During a first state only one phase is excited and it is excited with the large current. During the second state the one phase is excited with the large current and the other phase with the small current. During the third state both phases are excited with large currents. During the fourth state the one phase is excited with the small current and the other phase with the large current.

BACKGROUND OF THE INVENTION

The present invention relates to a driving circuit for a step motorwhich is suitable for carrying a rolled recording paper stepwise in arecorder such as a facsimile machine or printer.

In the case of a facsimile machine, for example, in order to record apicture on recording paper delivered from a feed roll, a step motor isused for a carrier mechanism to move the recording paper stepwise in thesubscanning direction.

FIG. 1 represents a step motor driving circuit employed in the prior artfor the carrier mechanism of the kind mentioned above. Each of theexciting coils 11-14 of a four phase winding wound on pole teeth of astator (not illustrated) has one end grounded. The other ends of thefour coils are connected to emitters of corresponding switchingtransistors 15-18 for on/off control of an exciting current for eachcoil. Collectors of the first and second transistors 15 and 16 areconnected to a power line 21 through a resistor 19, and collectors ofthe third and fourth trnsistors 17 and 18 are also connected to the samepower line 21 through a resistor 22. Control signals are inputted to thebases of the first to fourth transistors 15-18 from a control unit (notillustrated), thereby carrying out on/off control therefor.

When transistors 15-18 are rendered conductive in order, the coils 11-14are excited successively, and a rotor (not illustrated) then rotatescorrespondingly to obtain a stable position in a changing direction ofthe excitation. The resistors 19 and 22 are used in the circuit todecrease an electrical time constant. Therefore response to an inputsignal is quick, control of rotational speed is comparatively easy, andopen-loop control for which feedback is not required can be effected. Onthe other hand, an overshoot occurs at every step, and the carriersystem of the recorder is subject to vibrations. When vibrations occur,a satisfactory recording will not be obtainable from the recording papermoving to a given position until the vibrations attenuate to someextent. Thus, if the system operates to insure a still time of therecording paper at each subscanning point for better recording, it willnot be possible to obtain a high-speed recording. Another problem isthat in high speed recording the vibrations accumulate from shifting tothe next step operation before the complete attenuating of thevibrations of the step motor due to overshoot thus causing a step miss.

One proposal for solving the above problems includes a method to obtaina comparatively long still time for recording by moving the recordingpaper at the shortest possible time and reducing the vibrations of thestep response. This method is represented by FIG. 2. A negative-phaseclock (FIG. 2 at (b)) is generated between two positive-phase clocks(FIG. 2 at (a)), and an exciting step moving counter to the desiredrotation is mixed into the motor when a state is changed. Vibrations ofthe step response are thus reduced as shown in FIG. 2 at (c). However,according to this method, a rotation vector is generated counter to thedirection in which the rotor runs. Therefore the rotation torque of therotor is not fully utilized.

SUMMARY OF THE INVENTION

In view of the above circumstances, an object of the present inventionis to provide a step motor driving circuit utilizing most of the torquegenerated by the motor and capable of suppressing vibrations of the stepresponse.

The above object will be attained by providing two exciting currentswith their mean value changing with a trapezoidal waveform and their sumchanging stepwise and feeding them to exciting coils of each phase andcarrying a sequence of double 1-phase and 2-phase excitation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram representing a main part of a conventionalstep motor driving circuit;

FIG. 2 is an explanatory drawing representing a driving principle of astep motor suggested previously for reduction of vibrations of a stepresponse;

FIG. 3-FIG. 9 represent one preferred embodiment of the invention:

FIG. 3 is a circuit diagram of a step motor driving circuit,

FIG. 4 is a timing chart representing the sum of a clock signal and anexciting current flowing to both exciting coils,

FIG. 5 is a drawing representing a relation between a memory address anda logic signal appearing on each output terminal,

FIG. 6 is a waveform diagram representing a change in time of a voltageappearing on an output terminal of a detection voltage dividing transfercircuit,

FIG. 7 is a waveform diagram representing a change in voltages V_(A),V_(B) appearing on both ends of each current detection resistance whenaddress 0 of the memory is accessed,

FIG. 8 is an explanatory drawing representing a step operation of arotor, and

FIG. 9 is a characteristic drawing representing a carryingcharacteristic of a recording paper by a carrier mechanism using thestep motor driving circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 3 represents a step motor driving circuit for a two-phase motorhaving two phases A and B. In the circuit, an A-phase exciting coil 31Awound on a pole tooth of a stator (not illustrated) and four switchingtransistors 32A-35A constitute the first bipolar excitation basiccircuit 30A. A B-phase exciting coil 31B and four switching transistors32B-35B constitute likewise the second bipolar excitation basic circuit30B. The collectors of transistors 32A and 34A are connected to anexciting current feeding motor power terminal 41 and the emitters areconnected across the exciting coil 31A. The emitters of transistors 33Aand 35A are grounded through a first current detection resistance 42Aand are also connected to an input terminal 1 of a first detectionvoltage dividing transfer circuit 43A. The collectors of transistors 33Aand 35A are connected across the exciting coil 31A. The bases oftransistors 32A and 33A are connected to corresponding output terminals1 and 2 of an A-phase driver circuit 44A, and the bases of transistors34A and 35A are connected to corresponding output terminals 1 and 2 ofA-phase driver circuit 45A. The A-phase driver circuit 44A controls theflow of an exciting current to the exciting coil 31A in the directionindicated by arrow 46A. Input terminal 3 of circuit 44A is connectedwith the first output terminal 1 of a memory 47, which outputs acontrolling logic signal. The A-phase driver circuit 45A controls theflow of an exciting current to the exciting coil 31A in the directionindicated by arrow 48A. Input terminal 3 of circuit 45A is connectedwith the second output terminal 2 of the memory 47. Further, the inputterminal 2 of the first detection voltage dividing transfer circuit 43Ais connected with an output terminal 5 of the memory 47. When a logicsignal is applied to the voltage dividing transfer circuit 43A frommemory 47 a divided voltage is impressed on the first comparator 49Afrom an output terminal 3. A reference voltage input terminal 51 and arectangular wave generating circuit 53 through an AC breaking condenser52 are connected to a signal input terminal of the first comparator 49A,and a comparison output is supplied from the output terminal to an inputterminal 4 of the driver circuits 44A and 45A.

Peripheral circuits of the second bipolar excitation basic circuit 30Bare exactly the same in constitution as those of the first bipolarexcitation basic circuit 30A, and those circuits are denoted by asubscript B instead of A. However, an input terminal 3 of a B-phasedriver circuit 44B, an input terminal 3 of a B-phase driver circuit 45Band an input terminal 2 of the second detection voltage dividingtransfer circuit 43B are connected to corresponding output terminals 3,4 and 6 of the memory 47, respectively. The address for the memory 47 isspecified by a counter 55 receiving a supply of clock signals from aclock input terminal 54, and a given logic signal is outputted from theoutput terminals 1-6 of the memory 47.

A clock signal 61 read from a ROM (not illustrated) is supplied to theclock input terminal 54 of the step motor driving curcuit. The clocksignal consists of pulse groups having a period of 5 msec as shown inFIG. 4 at (a). Each pulse group consists of pulses P₁, P₂, P₃, and P₄separated by intervals of 1, 0.5, and 1 msec, respectively. The firstpulse, P₁, of the succeeding group is generated 2.5 msec aftergeneration of pulse P₄. The counter 55 is a hexadecimal counter, andwhen the clock signal 61 is supplied thereto, it accesses addresses 0 to15 of the memory 47 from the first pulse P₁ sequentially.

FIG. 5 represents the relationship of each address of the memory 47 tothe logic signals appearing on the output terminals 1-6. Let it beassumed that the step motor driving circuit is closed, and address 0 ofthe memory 47 is accessed. In the above state, a signal "1" is outputtedfrom the first, fourth and fifth output terminals 1, 4, 5, and a signal"0" is outputted from the other terminals 2, 3, 6. In this case, theA-phase driver circuit 44A and the B-phase driver circuit 45B, to whichthe signal "1" is supplied, are in operation; transistors 32A and 33A incircuit 30A and transistors 34B and 35B in circuit 30B are inconduction. In this state, an exciting current flows from the motorpower terminal 41 in the direction indicated by arrow 46A through theA-phase exciting coil 31A and in the direction indicated by arrow 48Bthrough the B-phase exciting coil 31B.

When the exciting current flows, voltages V_(A) and V_(B) proportionalto the current value appear across the first current detection resistor42A and the second current detection resistance 42B. The voltages V_(A)and V_(B) are impressed on the corresponding first and second detectionvoltage dividing transfer circuits 43A and 43B, respectively. When thesignal "0" is supplied to the input terminals 2, the detection voltagedividing transfer circuits 43A and 43B output the voltages V_(A) andV_(B), respectively, to the corresponding comparators 49A and 49B. Whenthe signal "1" is supplied to the input terminals 2, a voltage valueobtainable through multiplying the voltages V_(A) and V_(B) by 2/5 isoutputted to the corresponding comparators 49A and 49B. When the address0 of the memory 47 is accessed, the signal "1" is supplied only to theinput terminal 2 of the first detection voltage dividing transfercircuit 43A. Therefore the voltage 2/5 V_(A) is impressed on the firstcomparator 49A and the voltage V_(B) is impressed on the secondcomparator 49B.

The comparators 49A and 49B compare voltages inputted from the detectionvoltage dividing transfer circuits 43A and 43B, respectively, with thereference voltage, onto which a rectangular wave is superimposed. Thecomparators output control signals 62A and 62B, respectively, to thecorresponding driver circuits 44A, 44B, 45A and 45B only when thereference voltage plus the rectangular wave is larger than or equal tothat of the output of the voltage dividing transfer circuit. Otherwisethere will be no output. The driver circuits 44A, 44B, 45A, 45B willcome into operation only when the signal "1" is supplied from the memory47 and the control signal 62A or 62B is supplied from the correspondingcomparator 49A or 49B. As shown in FIG. 6, therefore, a voltage V_(O)outputted from the output terminal 3 of the detection voltage dividingtransfer circuits 43A or 43B rises quickly from the time of logic signalfor the address 0 is outputted and would climb up to a steady statevoltage E without further control. However, it is subjected to on/offcontrol by the rise and fall of the rectangular wave after it reachesequality with the reference voltage V_(R) subjected to comparison by thecomparator and is thus subjected to a constant current control.

In this case, since transfer circuit 43A has a logic signal 1 applied toits input terminal 2 and transfer circuit 43B has a logic signal 0applied to its input terminal 2, the voltages V_(A) and V_(B) will leveloff at different values. Voltage V_(B) will level off at the referencevoltage V_(R) because V_(B) appears at the output of circuit 43B as iscompared with the reference. On the other hand, the voltage value 2/5V_(A) appears at the output of circuit 43A and is compared to V_(R). Thevoltage V_(A) will level off when 2/5 V_(A) =V_(R). Thus the level offvalue of V_(A) =5/2 V_(R). This is shown in FIG. 7. Thus in the circuitdescribed, an exciting current of value A flows to the A-phase excitingcoil 31A at a balanced state, and an exciting current about 0.4 A flowsto the B-phase exciting coil 31B.

FIG. 8 represents the state wherein a rotor comes into a step operation.The top graph at (a) represents the stator teeth of the step motor,alternately wound with the A-phase and B-phase windings. The remaininggraphs in FIG. 8 at (b) to (f) represent the rotor teeth, at positionsrelative to the stator teeth, during successive steps of the motor. Asdescribed above, in the state where the address 0 of the memory 47 isaccessed, an exciting current 2.5 times of that applied to B-phaseexciting coil 31B flows to the A-phase exciting coil 31A, and thus theexciting energy generated on the A-phase will be larger than that on theB-phase. Therefore, as indicated by cross-hatched zones, a pole tooth 71of the rotor remains stationary between a pole tooth 72A of the statoron which the A-phase exciting coil 31A is wound and a pole tooth 72B₁ ofthe stator on which the B-phase exciting coil 31B is wound at a positionnear the former by an angle corresponding to the ratio of the excitingenergy. Said position becomes the first change balanced point.

When a given period of time passes after the step motor driving circuitis closed and the first pulse P₁ is supplied to the counter 55 as aclock signal 61, the memory 47 has the address 1 accessed instead of theaddress 0. Then, as shown in FIG. 5, the A-phase driver circuit 44Aoperates according to the divided voltage 2/5 V_(A) of the first currentdetection resistor 42A, and the B-phase driver circuit 44B is placed inan inoperative state. In this case, about 1A of exciting current flowsto the A-phase exciting coil 31A, and the A-phase only is excited. Inthis state, the rotor is magnetically attracted to step to a positionwhere the A-phase stator pole tooth 72A and the rotor pole tooth 71 faceeach other. (FIG. 8 at (c)).

When 1 msec passes after the address 1 of the memory 47 is accessed, thesecond pulse P₂ is supplied to the counter 55 as the clock signal 61,and the memory 47 has the address 2 accessed. In this state, the A- andB-phase driver circuits 44A and 44B and of the first detection voltagedividing transfer circuit 43A come into operation. In this case, about1A of exciting current flows to the A phase exciting coil 31A in thesame direction (indicated by arrow 46A) as before at the A-phase, and acurrent counter (as indicated by arrow 48B) to the exciting currentflowing to the B-phase exciting coil 31B at an initialized state (stateof address 0) flows at the B-phase. In this case, the rotor steps to aposition near the A-phase by a given angle with its pole tooth 71passing the A-phase stator pole tooth 72A in the direction indicated byarrow 73. (FIG. 8 at (d)).

When 0.5 msec passes after the address 2 of the memory is accessed, thethird pulse P3 is supplied to the counter 55 as the clock signal 61, andthe address 3 is accessed. In this state, about 1A of exciting currentflows to both the A- and B-phase exciting coils 31A, 31B. Excitingenergies generated on the A- and B-phases are equalized, the pole tooth71 of the rotor is attracted at an equal force by the pole teeth 72A,72B₂ of the A-phase and B-phase stators, and the rotor steps so as toposition just intermediately of the two. (FIG. 8 at (e)).

When 1 msec passes after the address 3 of the memory 47 is accessed, thefourth pulse P₄ is generated as the clock signal 61, and when theaddress 4 is accessed, a current flowing to the A-phase exciting coil31A becomes 2/5 times the current flowing to the B-phase exciting coil31B, and the exciting energy of the B-phase prevails over that of theA-phase. The rotor is consequently attracted more strongly to step inthe pole tooth direction (indicated by arrow 73) of the B-phase statorand comes to a standstill with its pole tooth 71 coming between the poleteeth 72A and 72B of the A-phase and B-phase stators at a position nearthe latter by a given angle. (FIG. 8 at (f)).

The step motor is driven by repeating a series of operations asdescribed above. In this case, exciting currents, the values of whichhave trapezoidal transitional form flow to the A- and B-phase excitingcoils 31A and 31B, and the sum indicates a stepwise change as shown inFIG. 4 at (b). Thus it can be appreciated by examining FIG. 5 along withFIG. 8 that each step of the motor is constituted by four states orsubsteps. In the first state one phase (e.g., A-phase in the case ofaddress 1) is excited with a large current and the other phase is notexcited. In the second state both phases are excited, but one phase isexcited with a large current (e.g., A-phase in the case of address 2)and the other with a small current (e.g., B-phase in the case of address2). In the third state both phases are excited with the large or fullcurrent. In the fourth state both phases are excited but the currentsapplied are the reverse of the second state, i.e., the one phase isexcited with the small current and the other is excited by the largecurrent. The mean exciting current of each phase approximates atrapezoid.

In a carrier mechanism using a step motor driven by the step motordriving circuit, the time in which recording paper is at a standstill isset by an interval between the fourth pulse P₄ and the next occurrenceof the first pulse P₁. In the embodiment, the interval is set at 2.5msec as shown in FIG. 4a.

FIG. 9 represents a positional state of a rolled recording paper in theabove carrier mechanism. Since a phase-shifted trapezoidal wave issupplied to an exciting coil of each phase from the step motor drivingcircuit, a still time T.sub.β long enough to allow the recording paperto move without inviting vibrations during its moving time T.sub.α isreasonably obtainable.

According to the invention, the step motor is controlled in a sequenceof double 1- and 2-phase excitation, therefore the circuit configurationis comparatively simplified and an economical system can be constituted.

Although concrete values of exciting current are given above for aspecific embodiment of the invention, it should be understood that thevalues can be changed according to the components of the step motor anda desired step response characteristic. Also, instead of superimposing arectangular wave on the reference voltage to be applied on thecomparator, a triangular wave or other wave form changing regularly canbe superimposed on the reference voltage.

What is claimed is:
 1. A step motor driving circuit for a step motorcomprising:(a) A-phase and B-phase exciting windings, said A-phase andB-phase windings being independent of each other; (b) first currentselection means for selectively applying currents having a first currentvalue, a second current value or zero value to said A-phase windings,said first and second exciting currents being of a predetermined ratio,said first being larger than said second, said first current selectionmeans varying said currents between said values with a graduallychanging waveform; (c) second current selection means for selectivelyapplying currents having said first current value, said second currentvalue or zero value to said B-phase windings, said second currentselection means varying said currents between said values with agradually changing waveform; and (d) control means for applying controlsignals to said first and second current selection means in a four statesequence of four successive control signal groups to cause the latter toexcite said A-phase and B-phase windings in the following sequence ofstates: excitation of one of said phase windings with a current of saidfirst current value and the other of said phase windings with a currentof zero value; excitation of said one winding with a current of saidfirst current value and the other winding with a current of said secondcurrent value; excitation of both phase windings with currents of saidfirst current value; excitation of said one winding with a current ofsaid second current value and excitation of said other winding with acurrent of said first current value, whereby each successive four stateexcitation constitutes one step of said step motor.
 2. A step motordriving circuit as claimed in claim 1 wherein said first and secondcurrent selection means further include means for reversing thedirection of said current applied to said exciting windings.
 3. A stepmotor driving circuit as claimed in claim 1 wherein for each successivestep of said step motor the applying of currents of said first andsecond current values by first and second current selection means tosaid one and said other windings during said four states is the applyingof currents in the reverse direction from corresponding currents excitedduring the four states of the previous step.
 4. A step motor drivingcircuit as claimed in claim 1 wherein the four successive states havetime periods that are not all equal.
 5. A step motor driving circuit asclaimed in claim 1 wherein said first current selection meanscomprises:A-phase current sensing means for providing an A-phase sensingsignal indicative of the current value through said A-phase windings;A-phase voltage dividing transfer circuit responsive to and dependentupon a control signal from said control means for selectively connectingsaid A-phase sensing signal or a divided portion of said A-phase sensingsignal to an output terminal thereof; A-phase comparator means, havingthe output terminal of said A-phase voltage dividing transfer circuitconnected to one input terminal thereof and a reference voltageconnected to a second input terminal thereof, for comparing the signalson said two input terminals and providing a logic activating signaloutput signal only when said reference voltage exceeds the signal onsaid one input terminal; and A-phase driver circuit means responsive tothe occurrence of said logic activating signal from said A-phasecomparator and a control signal from said control means for connectingsaid A-phase windings to a source of power to cause current to flow in afirst direction through said A-phase windings.
 6. A step motor drivingcircuit as claimed in claim 5 wherein said first current selectiondriving means further comprises:A-phase driver circuit means responsiveto the occurrence of said logic activating signal from said A-phasecomparator and a control signal from said control means for connectingsaid A-phase windings to a source of power to cause current to flow in asecond direction through said A-phase windings.
 7. A step motor drivingcircuit as claimed in any of claims 5 or 6 wherein said second currentselection means comprises:B-phase current sensing means for providing aB-phase sensing signal indicative of the current value through saidB-phase windings; B-phase voltage dividing transfer circuit responsiveto and dependent upon a control signal from said control means forselectively connecting said B-phase sensing signal or a divided portionof said B-phase sensing signal to an output terminal thereof; B-phasecomparator means, having the output terminal of said B-phase voltagedividing transfer circuit connected to one input terminal thereof and areference voltage connected to a second input terminal thereof, forcomparing the signals on said two input terminals and providing a logicactivating signal output only when said reference voltage exceeds thesignal on said one input terminal; and B-phase driver circuit meansresponsive to the occurrence of said logic activating signal from saidB-phase comparator and a control signal from said control means forconnecting said B-phase windings to a source of power to cause currentto flow in a first direction through said B-phase windings.
 8. A stepmotor driving circuit as claimed in claim 7 wherein said second currentselection means further comprises:B-phase driver circuit meansresponsive to the occurrence of said logic activating signal from saidB-phase comparator and a control signal from said control means forconnecting said B-phase windings to a source of power to cause currentto flow in a second direction through said B-phase windings.
 9. A stepmotor driving circuit as claimed in claim 8 wherein said referencevoltage consists of a constant value voltage with a periodically varyingvoltage superimposed thereon.
 10. A step motor driving circuit asclaimed in claim 8 wherein said control means comprises a memory meanshaving six control output terminals connected respectively to saidA-phase driver circuit, said A-phase driver circuit; said A-phasevoltage dividing transfer circuit; said B-phase driving circuit; saidB-phase driving circuit; and said B-phase voltage dividing transfercircuit; the state of the signals on said control output terminals beingdependent upon the address applied to said memory and the content ofsaid memory at said address.